Multi-antenna systems have been theoretically and practically proven to be capable of utilizing techniques such as precoding and/or beamforming to allow multiple users to access wireless resources and increase spectrum usage efficiency. Recently, it has been pointed out that if the number of antennas on the base station is more than four times the number of users, the spectrum usage efficiency will grow linearly with the increase of the number of users, which is called the massive antenna theory. In general, the number of base station antennas exceeding 64 can be called massive antenna. On the other hand, multiple base stations can be coordinated to jointly transmit data to the users to achieve performance equivalent to that of massive antenna. This is called a Multi-Cell Coordination (MCC) system. In the MCC system, all base stations are controlled by a coordination server. The best transmission mode can be selected according to the user conditions.
Since the clock sources of all the base stations in the MCC system are independent, there is a Carrier Frequency Offset (CFO) between the base stations. Other imperfections such as the sampling clock offset (SCO) caused by the CFO, the timing offset caused by the transmission delay, and the opposite linear phase in the downlink and uplink channels caused by the CFP, time-varying effects of RF response, etc., will result in inaccurate channel estimation. Inter-cell interference (ICI) and inter-user interference (IUI) may still occur after precoding, resulting in decreased system capacity. Therefore, proposing a multi-antenna system or a multi-base station system and a precoding method to solve the above problems is one of the issues the industry is currently focusing on.